All three curves of Figure 1 show that the majority of CH4 was released during the first 15 or 20 days of the test. This should not be used to estimate the rate of biogas release, nor the optimum holding time for a reactor. Remember, a small amount of substrate is fed to lots of hungry, healthy microbes under ideal conditions. The first burst of gas occurs as easily digestible material is removed. Gas production slows down as the remaining, harder to digest material is removed.